Boiler tube manufacturing uses a variety of different sized steel alloy tubes. The tubes are bent during the manufacturing process, often by drawing the tube around a round die having the desired bend diameter. Metal in the tube walls are displaced during the bending process. In addition, the inner circumference and the outer circumference may lose their roundness (circularity) during the bending process, resulting in the loss of roundness for sections of the tube.
Both, the U.S. American Society of Manufacturing Engineers (ASME) boiler code and the European EN boiler code regulations mandate the inspection of tube bends for several properties such as the minimum tube wall thickness, maximum tube wall thickness, flow area through the tube, ovality (out-of roundness) and flat spots. The inspection as per the ASME and the EN regulations mandate that the tube bend be first sectioned into several pieces to expose several cross-sections of the tube. The measurements of these cross-sections are taken with mechanical measuring equipment such as a vernier caliper or a micrometer. In some cases, an image of the cross-section of the tube is stamped onto a piece of paper (using ink) and the measurements are made on the image.
Both sets of boiler regulations (ASME and EN) specify calculations that must be performed based on the measurements, with accepted criteria for each calculated value. Typically, an inspection report is created after taking the cross-sectional measurements of the tube and performing the desired calculations. The inspection report details whether a particular bend sample failed or passed. This complete process may take several days if performed by an inspection laboratory.
In addition, the method of mechanical measurement is subject to human error and equipment error that occurs with extensive equipment use. It is therefore desirable to use a method that is quicker and that is less error prone.